Concatenating strings

I've been thinking about the seemingly simple task of writing a
va_arg-type function to concatenate arbitarily many strings.

My first thoughts violated the following principles:
1) never calculate the length of a string more than once;
2) never invoke realloc() multiple times if you can make do with a
single malloc().

As a result, I came up with the code below. This avoids the two pitfalls
above, but at the expense of needing three passes through the argument
list, which seems a bit clumsy.

Does anyone have any thoughts on the rights and wrongs of this?


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>

/* Returns the concatenation of the strings supplied as arguments. The
* final argument must be NULL.
* Caller should free() the returned pointer when done.
* In case of allocation failure, NULL is returned.
*/
char *concat(char *s, ...)
{
va_list ap;
unsigned n=1;
size_t *len, total=0;
char *t, *r;

/* pass 1: count arguments */
for(va_start(ap, s); va_arg(ap,char *); n++)
;

/* pass 2: get lengths */
if(!(len=malloc(n * sizeof *len)))
return NULL;
va_start(ap, s);
len[n=0]=strlen(s);
while(t=va_arg(ap, char *))
total+=(len[++n]=strlen(t));
va_end(ap);

/* pass 3: copy strings */
if(r=malloc(total+1)) {
memcpy(r, s, total = *len);
n=1;
for(va_start(ap, s) ; t=va_arg(ap, char *) ; total+=len[n++])
memcpy(r+total, t, len[n]);
va_end(ap);
}
free(len);
return r;
}

int main()
{
char *s;
s=concat("hello ", "world", " and ", "everyone", " in it!", NULL);
puts(s);
free(s);
}

re: Concatenating strings

On 2 Sep 2007 at 15:22, Big Brother wrote:
....
Oops,should be a va_end(ap); here, of course.
....

re: Concatenating strings

"Big Brother" <anonymous@dev.nullwrote in message
news:slrnfdll9b.c2g.spamoff@nospam.invalid...Here you seem to have forgotten the first string in calculating total.
This was hard to spot, because of all the fancy assignments in expressions.
No terminating nul. Also, aren't you copying string t on the first pass?
That's set to NULL at this point.

re: Concatenating strings

Big Brother <anonymous@dev.nullwrote:
# I've been thinking about the seemingly simple task of writing a
# va_arg-type function to concatenate arbitarily many strings.
#
# My first thoughts violated the following principles:
# 1) never calculate the length of a string more than once;
# 2) never invoke realloc() multiple times if you can make do with a
# single malloc().

You're free to do whatever you want, but as far as practical code,
if you realloc by a constant factor (say double the string length
every realloc) instead of by a constant amount, the running time
is linear in the the length of the strings for any malloc you're
likely to encounter.

--
SM Ryan http://www.rawbw.com/~wyrmwif/
Death is the worry of the living. The dead, like myself,
only worry about decay and necrophiliacs.